Available evidence suggests that global warming may lead
to substantial changes in mean annual streamflows, seasonal distributions of
flows, and the probabilities of extreme high- or low-flow conditions (Leavesley,
1994; Cubasch et al., 1995; Mearns et al., 1995; Trenberth and
Shea, 1997). Runoff characteristics may change appreciably over the next several
decades, but in the near term, the hydrological effects of global warming are
likely to be masked by ongoing year-to-year climatic variability (Rogers, 1994;
Miller, 1997; Matalas, 1998). There is some evidence that the intensity of rainfall
events may increase under global warming, as a result of increases in the precipitable
water content of the atmosphere (IPCC, 1996; Trenberth and Shea, 1997). This
may increase flooding risks in some watersheds. Hydrological changes cannot
yet be forecast reliably at the watershed scale, although numerous studies have
addressed the potential effects of warming scenarios on water availability in
North America (e.g., Mortsch and Quinn, 1996; Melack et al., 1997; Moore
et al., 1997; Mulholland et al., 1997; Woodhouse and Overpeck,
1998; Wilby et al., 1999; Wolock and McCabe, 2000). Figure
15-1 summarizes some possible regional hydrological and ecological impacts
of climate change identified in recent analyses.

In general, there is greater confidence in projections of seasonal shifts in
runoff and related hydrological characteristics than there is in projections
of changes in annual runoff. Regional patterns of precipitation change are highly
uncertain. Runoff changes also will depend on changes in temperatures and other
climatic variables. Warmer temperatures may cause runoff to decline even where
precipitation increases (e.g., Nash and Gleick, 1993). Changes in vegetation
characteristics will have further, complex impacts on streamflows (Callaway
and Currie, 1985; Rosenberg et al., 1990; Riley et al., 1996).
Wolock and McCabe (2000) computed annual runoff projections for the 18 major
water-resource regions of the continental United States for two GCM scenarios
used in the U.S. National Assessment. They found very little agreement between
the modelsthe Canadian Centre for Climate Prediction and Analysis (CCC)
model and the Hadley Centre for Climate Prediction and Research (HAD) modelregarding
the direction of change in average annual runoff. In addition, most of the projected
changes for the next century fell within the range of current variability.

Projections of shorter snow accumulation periods appear to be more robust.
Many studies of snowmelt-dominated systems show similar seasonal shifts to greater
winter runoff and reduced summer flow (e.g., Cooley, 1990; Lettenmaier and Gan,
1990; Rango and Van Katwijk, 1990; Duell, 1992, 1994; Lettenmaier et al.,
1992, 1996; Rango, 1995; Melack et al., 1997; Fyfe and Flato, 1999; Wilby
et al., 1999). In mountainous areas of western North America, small high-elevation
catchments may contribute the bulk of the flow of major river systems (Schaake,
1990; Redmond, 1998). Although some models predict decreases in snowpack, records
from at least one long-term alpine site in the Rocky Mountains show an increase
in annual precipitation since 1951 (Williams et al., 1996). Earlier melt-off
in combination with either lower or higher snowpack will tend to increase winter
or spring flows and reduce summer flows. Warmer temperatures could increase
the number of rain-on-snow events in some river basins, increasing the risk
of winter and spring floods (Lettenmaier and Gan, 1990; Hughes et al.,
1993; Loukas and Quick, 1996). Lower summer flows, warmer summer water temperatures,
and increased winter flows are results on which many of the regional ecological
impacts identified in Figure 15-1 are based.

Studies based on climate change scenarios from older versions of GCMs that
did not include aerosol effects suggest reductions in streamflow and lake levels
in many Canadian watersheds, despite scenario increases in annual precipitation
(Hofmann et al., 1998). Bruce et al. (2000) examined a variety
of newer evidence, including temperature and precipitation changes projected
by transient runs of seven different atmosphere-ocean GCMs (AOGCMs) with business-as-usual
greenhouse gas (GHG) and aerosol increases. They conclude that many areas of
Canada, including southwestern Canada and the Great Lakes region, could experience
" reduced total flow, lower minimum flows and lower average annual
peak flow" (Bruce et al., 2000).

Open-water evaporation is an important part of the water balance of the North
American Great Lakes. Increased evaporation as a result of warmer water temperatures
therefore would likely affect future lake levels and outflow into the St. Lawrence
River (Mortsch and Quinn, 1996; Mortsch et al., 2000). Most analyses
for the Great Lakes suggest declines in lake levels and outflows (Mortsch and
Quinn, 1996; Chao, 1999). Chao (1999), for example, examined 10 different transient
GCM scenarios (without aerosols) for IPCC decades 2 and 3 and concludes, "In
general the decrease in inflows under all the GCM scenarios result in negative
impacts to hydropower, navigation and coldwater habitat, and positive ones to
shoreline damages." Mortsch et al. (2000) compared such early results
to results based on transient runs of the CCC model and the HadCM2 model, both
of which include aerosol impacts. Whereas the CCC model run suggests declines
in lake levels and outflows comparable to the earlier doubled-CO2
runs (e.g., a 1.01-m decline in the level of Lakes Michigan and Huron by 2050),
the HadCM2 model indicates the possibility of a small rise in lake levels and
outflows (0.03-m rise in Lakes Michigan and Huron by 2050). Caution is required
in interpreting these results because there is substantial uncertainty regarding
future sulfate emissions, and projections of aerosol concentrations have declined
considerably since these runs were performed (Carter et al., 2000).

Arid environments are characterized by highly nonlinear relationships between
precipitation and runoff. Thus, streamflows in the arid and semi-arid western
portions of North America will be particularly sensitive to any changes in temperature
and precipitation (Schaake, 1990; Arnell et al., 1996; Kaczmarek et
al., 1996). Rivers that originate in mountainous regions will be particularly
sensitive to winter precipitation in the headwaters, regardless of conditions
in the downstream semi-arid zone (e.g., Cohen, 1991). In addition, " severe
flood events may be more damaging in drier climates where soils are more erodible "
(Arnell et al., 1996).

Little research attention has been given to the possible impacts of climate
change on sediment transport and deposition, which could affect aquatic ecosystems,
reservoir storage capacity, potential flood damages, and the need for dredging
operations. However, projected increases in the intensity of precipitation events
could contribute to increased erosion and sedimentation in some areas (Mount,
1995).